There has been known an oil-free or oilless type screw compressor which is operable to compress air by means of a pair of male and female screw rotors rotatable in a contactless and oilless manner. The oil-free screw compressor includes a compressor body for air compression and is provided with a cooling unit for cooling compressed air because the compressed air discharged from the compressor body has high temperatures. The compressed air discharged from the compressor body flows through pipes running through these cooling unit and compressor unit so as to be discharged to the outside of the compressor unit.
The above oil-free screw compressor includes a suction throttle valve for controlling air intake into the compressor, a piston assembly for operating the suction throttle valve, and a structure for supplying a working pressure to the piston assembly. The screw compressor is further provided with a structure for draining, to the outside of the compressor, condensate resulting from the operations of compressing the air and cooling the compressed air.
Patent Literature 1 (JP-A No. 63(1988)-61780) discloses a capacity controller for a multi-stage compressor, which will be described hereinlater.